Motor height for a displacement hull

Discussion in 'Outboards' started by boat fan, Jan 29, 2016.

  1. boat fan
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    boat fan Senior Member

    Hello everyone.

    I could do with some advice regarding the placement of an outboard motor on a displacement hull.The motor will be a Yamaha high thrust or Bigfoot four stroke.

    The hulls in question are two 40 ft 12 M ) long pontoon hulls , 3 ft wide and draft is 12 inches. This is a photo with the projected waterline at 12 inches marked in red.

    [​IMG].

    The motor will be mounted on a centered pod suspended below the 2x6 deck beams.


    I am trying to determine at what height the cavitation plate should be in relation to the projected waterline.
    I know that there are a lot of variable , and it`s not an exact science , but I`m just looking for an approx. " best guess " , a place to start , and then adjust with a jack plate.

    I have searched far and wide , but most of the information seems to be about fast planing type hulls.Thanks.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't follow this part:
    "The motor will be mounted on a centered pod suspended below the 2x6 deck beams."
    Is it going to be in a well ?
     
  3. boat fan
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    boat fan Senior Member

    Hi Mr. Efficiency , and thank you for the quick reply.

    The hulls are connected by 24 2x6 deck beams.

    Below them there will be a box mounted along the centre line .

    Like this , but suspended off the deck beams , as there is no " stern " to speak of , to bolt onto..

    [​IMG]
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The first thing that comes to my mind is to select an engine with a leg that is streamlined above the cav plate for at least 6 inches, not all are, longer legs more likely to be. That way you won't suffer too much retardation from an over-immersed motor, which shouldn't be a problem (deep immersion) at lowish speed. And if the engine will be right aft, allowance for pitching has to be made.
     
  5. boat fan
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    boat fan Senior Member

    Hi again and thank you .

    Pitching is no issue , it`s a river boat.
    ( Murray River Murraylands South Australia.

    [​IMG]

    [​IMG]

    These are the black water tanks , the pod will be hung similar to this.

    [​IMG]

    The engine I have is a long shaft ( 20 inch )

    [​IMG]

    Just need a good guess how deep to place the cav plate ?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously variations in load, and consequently waterline, have to be considered. You have the option of installing a fairing ahead of the leg, about the same width as the engine leg, on the bottom of your pod, which should allow you to put it deeper without much extra drag, and the need for the jacking plate will depend on the range of draught. And in any application where there is a risk of water dropping below the cav plate momentarily even, you don't want an alloy prop, they "let go" too easily.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Hi Boat Fan,

    First of all, a small note on the terminology - the plate in question is called anti-ventilation plate and not cavitation plate. It prevents the atmospheric air from being sucked down to the area of low pressure created by the prop blades.

    Now, you will be travelling at low speeds - so the engine-leg drag will not be an issue for you. All you need to assure is that the water pickup is always flooded and that no ventilation occurs. So the correct height will be the one which places the anti-ventilation plate approxim. 1"-2" below the water surface at maximum speed. Since you don't know where the water surface will actually be (its shape changes as the boat speed and loading vary), your best bet is to mount the outboard to an adjustable jack-plate like one of these: http://www.jack-plate.com/, and then to do a few test-runs to see if the plate is at correct heght, eventually correcting it.

    Cheers
     
  8. PAR
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    PAR Yacht Designer/Builder

    Rather than spending a bunch on a jack plate, just mount it where you think it needs to be, assuming good bury for the water pickup. Try it out and adjust the height once you've got some sea trials under the keel. You can move it up or down as needed, eventually settling on a height that's suitable.
     
  9. boat fan
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    boat fan Senior Member

    Hi Guys , and thank you !



    Daquiri , good advice , I will try the 1- 2 inches below the anticipated water line. It`s a start , which is what I was after. Thank you.

    Re : terminology , I admit that I `m no expert on outboards , I`m actually easily confused because I don`t know much about them ...so , this was taken from Yamaha`s own manual ? :

    I thought that was what you call the anti - ventilation plate ?
    Maybe different terms in different parts of the world ?

    Hi Paul ,

    Can do what you suggest , it should not be too difficult to make adjustments the way you say.

    Thanks guys , I have a start !:D
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Yes, the makers themselves still call them cavitation plates. Plenty of misnomers in the world. The main thing is people know what is being referred to, not why it got a certain name. But I would absolutely recommend you ditch the alloy prop, in this kind of application, you will get a greater range of useable height with stainless steel.
     
  11. daiquiri
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    daiquiri Engineering and Design

    I am surprised that Yamaha has used such a gross misnomer. It is an anti-ventilation plate. Ventilation and cavitation are two very different physical phenomena.

    Cheers
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The problem there is that the term "cavitation plate" has been in force, for many decades, however erroneously, and no-one wants to be the first to confuse buyers by changing it, and cause congestion on their customer service hotline !
     
  13. boat fan
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    boat fan Senior Member

    Ha ha ....not me ...I have you guys to ask :D
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    a 25" motor would be better here, imo. Less chance of a mighty fountain shooting upward from the thing when it is running deep. You have a 5'' slim-line spacer with the splash plate above that, in the XL versions of most engines. Failing that, make the pod height adjustable, rather than adjusting the engine height on the pod. You don't want the water stream hitting the motor above the upper splash plate.
     

  15. boat fan
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    boat fan Senior Member

    Hmm....interesting comment.

    I was at the local slipway today to check out the pontoon house boats and asked Brett ( he owns the slipway ) regarding outboard mounting heights , one of the things he mentioned was that almost all the motors they fit are actually short shaft.......
    but then again , they don`t bother to sweep up the run at the sterns either , they just chop them off square , and drag that stern .... like it is done on the US style pontoon boats.
    Cheaper for them I guess.

    It`s all a steep learning curve for me though ....Thanks for all your help.
     
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